From fe40d6b804f2312e379787fae8a6df329c37e04b Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonygarnockjones@gmail.com>
Date: Sun, 21 Oct 2018 13:32:06 +0100
Subject: [PATCH] Present-to-absent check is necessary in assertion removal
 handler

---
 ijs/src/skeleton.js      | 13 ++++++++-----
 imperative/HOWITWORKS.md |  5 +++--
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/ijs/src/skeleton.js b/ijs/src/skeleton.js
index 32e4460..344bf45 100644
--- a/ijs/src/skeleton.js
+++ b/ijs/src/skeleton.js
@@ -204,11 +204,14 @@ function add_to_handler(h, vs) {
 function del_from_cont(c, v) { c.cachedAssertions = c.cachedAssertions.remove(v); }
 function del_from_leaf(l, v) { l.cachedAssertions = l.cachedAssertions.remove(v); }
 function del_from_handler(h, vs) {
-  h.cachedCaptures = Bag.change(h.cachedCaptures, vs, -1).bag;
-  h.callbacks.forEach((cb) => {
-    cb(EVENT_REMOVED, vs);
-    return true;
-  });
+  let net;
+  ({bag: h.cachedCaptures, net: net} = Bag.change(h.cachedCaptures, vs, -1));
+  if (net === Bag.PRESENT_TO_ABSENT) {
+    h.callbacks.forEach((cb) => {
+      cb(EVENT_REMOVED, vs);
+      return true;
+    });
+  }
 }
 
 Index.prototype.adjustAssertion = function(outerValue, delta) {
diff --git a/imperative/HOWITWORKS.md b/imperative/HOWITWORKS.md
index 898df22..7ad4761 100644
--- a/imperative/HOWITWORKS.md
+++ b/imperative/HOWITWORKS.md
@@ -444,8 +444,9 @@ check followed by zero or more (move, check) pairs.
         del_leaf (leafcache, _) v = leafcache -= v
         del_handler (bag, f_table) vs =
           bag[vs] -= 1
-          for f in f_table
-              f "-" vs
+          if vs not in bag then
+              for f in f_table
+                  f "-" vs
 
 **Definition.** The procedures `addAssertion` and `removeAssertion`
   install and remove an assertion `v` into the given index,